1. Field of the Invention:
The present invention relates to a magnetic recording medium and a magnetic recording apparatus. In particular, the present invention relates to a magnetic recording medium based on the perpendicular magnetic recording system suitable for the high density recording, and a magnetic recording apparatus.
2. Description of the Related Art:
In recent years, as the information oriented society is advanced, it becomes possible to process, for example, not only the character information but also the voice information and the image information at high speeds. One of the apparatuses capable of processing the information as described above at the high speed is a magnetic recording apparatus which is installed, for example, to a computer. The magnetic recording apparatus is being developed in order to realize a further small size while improving the recording density.
In general, the magnetic recording apparatus comprises a plurality of magnetic disks which are rotatably installed on a spindle. Each of the magnetic disks comprises a substrate and a magnetic film (appropriately referred to as “recording layer” as well) which is formed on the substrate. Information is recorded by forming magnetic domains having specified directions of magnetization in the magnetic film. Conventionally, the direction of magnetization subjected to the recording in the magnetic film is the in-plane direction of the magnetic film. This system is called “in-plane recording system”. The realization of the high density recording on the magnetic disk based on the in-plane recording system can be achieved by thinning the film thickness of the magnetic film, providing minute grain diameters of magnetic crystal grains which constitute the magnetic film, and reducing the magnetic interaction between the respective magnetic crystal grains. However, when the magnetic crystal grains are made to be fine and minute and the magnetic interaction between the magnetic crystal grains is reduced, then the thermal stability of the recorded magnetization is deteriorated. In order to solve this problem, a magnetic disk based on the perpendicular recording system has been suggested.
In the perpendicular recording system, the direction of magnetization of the magnetic domain in the magnetic film is perpendicular to the film surface to perform the recording. When this system is used, then the interstice between the adjoining recording bits is magnetostatically stabilized, the thermal stability is improved, and the recording transition area becomes sharp. Further, when a layer formed of a soft magnetic material (hereinafter referred to as “soft magnetic back layer”) is added between the substrate and the recording layer of the magnetic disk based on the perpendicular recording system, then the magnetic field, which is applied to the recording layer during the recording of information, can be converged, and it is possible to perform the recording on a magnetic material having higher magnetic anisotropy. It is possible to perform the recording at higher recording densities, because the magnetization of the magnetic material having the high magnetic anisotropy has high thermal stability.
A CoCr-based alloy is used for the recording layer of the magnetic disk based on the in-plane recording system described above. It is tried to similarly apply the CoCr-based alloy to the recording layer of the magnetic disk based on the perpendicular recording system as well. A two-phase separation structure, which is composed of the ferromagnetic crystal grains having a high Co concentration and the non-magnetic crystal grain boundary having a high Cr concentration, is formed in the recording layer formed of the CoCr-based alloy. The magnetic interaction between the ferromagnetic crystal grains can be shut off by the non-magnetic crystal grain boundary. Accordingly, the low noise of the medium, which is required for the high density recording, has been hitherto realized.
However, in order to realize the recording at higher densities, it is necessary to further reduce the magnetic interaction between the crystal grains. There is a method for solving this problem, in which the crystal grain boundary is made of oxide in the recording layer formed of the CoCr-based alloy. Such a method is carried out by adding an oxide to a sputtering target, or by forming a film of the recording layer in an oxygen gas atmosphere. A medium, which comprises the recording layer composed of the CoCr-based oxide obtained by such a method, has an oxide granular structure in which the magnetic crystal grains of the recording layer are surrounded by the oxide. The magnetic interaction between the magnetic crystal grains is further reduced by the oxide granular structure, and it is possible to further reduce the medium noise of the magnetic recording medium.
When the medium, which comprises the recording layer composed of the CoCr-based oxide, is manufactured, it is necessary that the crystalline orientation of the recording layer is controlled so that the easy axis of magnetization of the recording layer formed of the CoCr-based oxide is directed in the direction perpendicular to the film surface. For this purpose, an underlayer or underlying base layer is used. The crystalline structure of the CoCr-based oxide is the hcp (hexagonal close-packed lattice) structure, and the easy axis of magnetization thereof resides in the c-axis direction. Therefore, in order that the c-axis of the CoCr-based oxide is oriented in the direction perpendicular to the film surface, it is necessary to provide the underlayer which has the same hcp structure as that of the CoCr-based oxide. The element, which has the same hcp structure as that of the crystalline structure of the CoCr-based oxide, includes Ti, Ru, and alloys thereof. An exemplary magnetic recording medium, which has an underlayer formed of such an element, has been suggested. In this magnetic recording medium, a CoPtCrO magnetic layer is used for a recording layer, and the underlayer is obtained by combining a first underlayer which is mainly composed of Ti and a second underlayer which is mainly composed of Ru as disclosed in Japanese Patent Application Laid-open No. 2001-6158 (p. 3, FIG. 1). Another magnetic recording medium based on the in-plane recording system has been also disclosed in Japanese Patent Application Laid-open No. 2002-208126 (pp. 4 to 5, FIGS. 1 to 2), in which a CoPtCr alloy magnetic film is used for a recording layer, and a CoCrRu film is used for an underlayer.
According to the knowledge of the present inventors, it is considered that the improvement in the crystalline orientation of the recording layer is limited as a matter of course when the underlayer is formed with a single layer of film mainly composed of Ti. For example, in Japanese Patent Application Laid-open No. 2001-6158, the crystalline orientation of the recording layer is improved by combining the first underlayer which is mainly composed of Ti and the second underlayer which is mainly composed of, for example, Ru. That is, it is necessary that a plurality of underlying base films are combined in order to further improve the crystalline orientation of the recording layer. Therefore, the film thickness of the underlayer is thickened in the case of the magnetic recording medium as disclosed in Japanese Patent Application Laid-open No. 2001-6158. However, the soft magnetic back layer is provided between the underlayer and the substrate in order to improve the recording characteristics in the case of the magnetic recording medium based on the perpendicular recording system. When the plurality of underlayers are used in order to improve the crystalline orientation of the recording layer, an accidental problem arises such that the distance between the soft magnetic back layer and the recording layer is increased to cause a phenomenon in which the magnetic coupling between the both is inhibited, and the recording characteristics are deteriorated.
On the other hand, the magnetic recording medium disclosed in Japanese Patent Application Laid-open No. 2002-208126 is a magnetic recording medium based on the in-plane recording system. The c-axis of the crystal of the CoPtCr alloy magnetic film containing oxygen as its magnetic recording film is directed in the in-plane direction. Therefore, it is essentially difficult to perform the perpendicular magnetic recording in which the areal recording density is high. Japanese Patent Application Laid-open No. 2002-208126 describes that only the CoCrRu layer is insufficient as the underlayer for the recording layer, and it is essential to stack this layer and a layer which is composed of at least one of Ru, Re, and Os and which contains oxygen. It is taught that a plurality of underlayers are necessary. The magnetic recording medium as disclosed in Japanese Patent Application Laid-open No. 2002-208126 is based on the in-plane recording system. Therefore, it is unnecessary to decrease the distance (so-called magnetic spacing) between the magnetic pole of the writing head and the surface of the soft magnetic back layer. Such a magnetic recording medium does not solve the problem inherent in the perpendicular magnetic recording system, i.e., the problem in which the steepness of the slope of the writing magnetic field distribution disappears as the distance between the recording layer and the soft magnetic back layer is increased, and it becomes impossible to form small recording bits, resulting in the failure in the high density recording.
In view of the above, the present inventors have obtained the following knowledge. That is, it is necessary to develop a new underlayer which has a thin film thickness and which remarkably improves the orientation of a recording layer even with only one layer in order to realize the further high density recording while suppressing the deterioration of the recording characteristics in a magnetic recording medium based on the perpendicular recording system comprising the recording layer composed of CoCr-based oxide. Thus, the present invention has been made.